Possibly the most basic limitation in the transmission of information is the limitation caused by finite bandwidth transmission circuits. For example, in the field of radio telephony and conventional telephony, due to the cost of wide bandwidth circuits and limitations in radio spectrum, voice channels are generally restricted to 300 to 3,000 Hz even though components of speech extend below 100 Hz and above 5,000 Hz. Actually, some speech sounds, such as S's and H's, are readily confused when transmitted over conventional telephone circuits because the spectrum of such sounds exceeds the high frequency limit of 3,000 Hz.
Furthermore, if speech sounds are to be natural, pleasant to listen to, and not fatiguing, components below 300 Hz must be audible.
A prior invention, U.S. Pat. No. 3,684,838, discusses one means for improving the low frequency characteristics of a telephone line. There are a number of other techniques, such as U.S. Pat. Nos. 2,576,115 and 3,696,298 which teach one how to utilize two low quality telephone circuits to improve speech and music quality by approximately doubling the bandwidth of the system. The use of two lines is, of course, more expensive than one line.
Systems have also been developed for expanding or contracting the time duration of speech sounds and while such techniques are most useful when it is necessary to conform to tight time schedules, say, during a news broadcast, such systems alter the naturalness and may even degrade intelligibility.
It is also well known that one can transmit music or speech or video information, at say half speed, if one is willing to take twice the time to transmit the intelligence. There are two limitations to such a technique:
(1) They are not "real" time systems and therefore are not suitable for use with normal conversations; and,
(2) WHEN THE TRANSMISSION SPEED IS REDUCED TO TWO-TO-ONE THIS ALSO REDUCES THE FREQUENCY OF THE LOW FREQUENCY COMPONENTS BY 2-TO-1 EXACERBATING THE PROBLEM OF LOW FREQUENCY TRANSMISSION.
Numerous patents, including U.S. Pat. Nos. 1,836,824, 1,948,973, 2,014,081 and 2,306,425 disclose means for reducing the bandwidth of toll grade voice circuits.
Some of these techniques cause perceptible distortion or upset the temporal relationship of voice or video signals.
One system which has been successfully used to improve the spectrum utilization of communications channels in the TASI (Time Assignment Speech Interpolation) system. In this system a number of telephone circuits share a lesser number of transmission channels. The improvement in spectrum utilization is accomplished by assigning a channel to a circuit only when short term analysis indicates speech is present in the circuit. Thus, channel space is not wasted during idle speech periods.
One of the most difficult problems facing designers of TASI systems is the determination of the presence of speech. As pointed out in the textbook, "Introduction to Signal Transmission", McGraw-Hill Book Company, New York, 1970, by W. R. Bennett, pages 74 to 76, setting the threshold of a TASI speech presence detector is a compromise between loss of weak speech sounds and false operation by background noise. While the procedures described in U.S. Pat. No. Re. 27,202 and patent application Ser. No. 693,716 can be used to ease this problem, it is still somewhat difficult to accommodate the large differences in speech levels often experienced in practical telephone operation.
While the invention is subject to a wide range of applications, it is especially suited for transmission of telephone signals and will be particularly described in that connection.